A volcano is a terrain feature created at worldgen. Embarking to a volcano site is the easiest way to obtain magma, as a volcano has magma exposed to the surface rather than dozens or hundreds of z-levels below ground. The volcano will be consistently shaped all the way down to the bottom of the map.

Volcanoes do not explode, erupt, nor bellow noxious fumes, however, the level of magma within can fluctuate. For now volcanoes are simply very large, very hollow pillars of obsidian with magma inside. They count as subterranean lava for the purpose of spawning creatures, so magma natives like magma crabs and fire imps will freely roam through the volcano's interior, and may even leave the crater and roam the surface if they happen to path to it.

Volcanoes are one-embark-tile features, meaning that they can be surrounded by any biome. On the Region embark map, they are represented as a red raised caret (or circumflex) (^), the same symbol as a weapon trap in-game. On the Local embark map they are represented as two tildes (≈), the same symbol that represents magma in-game. On a standard world map, there will generally be only a few volcanoes.

Volcanoes tend to appear in mountainous igneous extrusive biomes, which means you may be hard pressed to find flux, coal or wood. You won't need it for fuel, but you will still need it for steel making. If you don't feel like searching hundreds of worlds for a volcano next to sedimentary stone or in a forest, your best bet may be to breach the first cavern for wood (or make tree farms underground once the caverns are opened up), or trade for wood, processed coke or charcoal. A way to use advanced world generation to mitigate this can be found here.

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Mining into a warm surface will result in your miner being overrun with the magma before completing the task. Digging a channel beside the warm rock then mining directly into it can safely open a magma flow for redirecting. Another way is to simply channel the warm rock from above.

A more complicated but fool proof method is to tap the tube from one level below, while your miner is protected by an overhead bridge. First, channel a hole next to the place you want to breach the volcano, creating a hole and a ramp. Then, place a one tile magma-safe bridge over the ramp. Third, forbid the area the bridge is in -- blocking the area off with a forbidden door, or a contraction, depending on how your fortress works. Then mine the area out from the ramp below.

Any magma in a volcano that is drained will replenish itself over time. This can make tunneling into the lip of a volcano especially fun. The best way to control volcano magma is to pump it out.

If you really want a volcano next to a sedimentary layer, you can adjust advanced world generation parameters to maximize your chances: increase the minimum number of volcanoes, increase the X and Y variance of volcanism, and set the terrain mesh weights for volcanism to use only the lowest and highest ranges.

You can instantly discover a magma sea and Candy if you simply cause a magma-safe cave-in somewhere above the lava. This can be accomplished by creating a floor, then creating a floor out from there with magma safe material, like obsidian. Remove the supporting floor so the obsidian will drop into the magma, and it will never melt, allowing you to use it as a "scout" to find veins of adamantine and a magma sea.